1. Field of the Invention
The present invention relates to an active matrix substrate which is provided with thin film transistors (hereinafter called "TFT") as switching elements and storage capacitors, and is typically used for an active matrix liquid crystal display device.
2. Description of the Prior Art
An active matrix liquid crystal display device comprises an active matrix substrate and a counter substrate which face each other with a liquid crystal layer sandwiched therebetween. A known active matrix substrate is shown in FIGS. 6 and 7, which comprises a plurality of gate buses 3 arranged in parallel and a plurality of source buses 8 arranged in parallel and perpendicular to the gate buses 3, thus forming a matrix.
TFTs 13 are arranged in a matrix on a substrate 1 with each allocated in a pixel area which is defined by the adjacent gate buses 3 and the adjacent source buses 8. The TFT 13 comprises a gate electrode 2 and a source electrode 7 which are connected to the gate bus 3 and the source bus 8, respectively. The TFT 13 also comprises a drain electrode 9 which is connected to a pixel electrode 10. The pixel electrode 10 which is also disposed in each pixel area defines a room for storing liquid crystal together with a counter electrode disposed on the counter substrate. Further, part of the pixel electrode 10 is overlapped with part of the gate bus 3 with an insulating film 5 interposed therebetween so as to form a storage capacitor 12. In this way, the storage capacitors 12 are arranged in a matrix, like TFTs 13, on the substrate 1.
The whole or part of the pixel electrode 10 is open to the outside through an opening of a light shield film disposed on the counter substrate. The open portion of the pixel electrode 10 serves as an effective display portion. The TFTs 13, the gate buses 3, the source buses 8 and the like are covered with the light shield film.
In a display device having an active matrix substrate of the above structure, a high precision display can be achieved by reducing the size of the pixel electrode 10 without lowering the ratio of the open portion of the pixel electrode 10 through the light shield film to the whole area of the substrate, or the aperture. For this purpose, in addition to reducing the size of the pixel electrode 10, it is required to reduce the area on the substrate which is covered with the light shield film. This can be effected, for example, by reducing the widths of the gate bus 3 and the source bus 8 and reducing the sizes of the TFT 13 and the storage capacitor 12.
Reducing the size of the storage capacitor 12 while securing the storage capacitance thereof has been conventionally performed by the following methods:
(a) reducing the thickness of the insulating film 5 of the storage capacitor 12,
(b) using a material with a high dielectric constant for the insulating film 5, and
(c) increasing an effective area for capacitance by forming a stacked structure for the storage capacitor 12.
However, when the area of the storage capacitor 12 is reduced according to the above methods, the following problems arise, respectively. In the above method (a), when the insulating film 5 is made thin, the distance between the pair of electrodes, that is, the distance between the pixel electrode 10 and the gate bus 3 overlapped with each other with the insulating film 5 interposed therebetween, is shortened, causing the increase of leakage current and the lowered withstand pressure. Therefore, the thickness of the storage capacitor 12 can not be so reduced, and as a result, it is practically not possible to reduce the area of the storage capacitor 12 by this method.
In the above method (b), a material such as Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5 having relative dielectric constants of about 10 and about 30, respectively, may be used. However, the insulating film 5 formed of such a material is inferior in quality and results in a large leakage current, compared with that formed of a material generally used such as SiO and SiN having relative dielectric constants .epsilon. of about 4 and about 7, respectively. As a result, the insulating film 5 made of Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5 is required to be thicker than that made of the material generally used. Therefore, this method is not so effective since the effective capacitance can not be increased.
In the above method (c), the area can be reduced by forming a stacked structure for the storage capacitor. However, the manufacturing process of such a stacked structure is complicated, and further since a sharp difference in thickness between the portion of the stacked structure and other portions of the substrate is formed, buses formed over the different thicknesses may be easily cut off.
The objective of the present invention is to solve the above problems and provide an active matrix substrate which comprises a storage capacitor occupying a small area but having a sufficient capacitance, thereby achieving a high precision display.